Musannaf Ibn Abi Shayba 3:5937

Waki’ narrated to us, he said: Hisham al-Dastawa’i narrated to us, from Qatadah, from ‘Ata’, from ‘Ubayd ibn ‘Umayr, from ‘A’ishah, she said: The prayer of the signs is six bowings (ruku’) in four prostrations (sujud).